1. Field of the Invention
The invention relates generally to receivers and more particularly to a GPS receiver having a carrier ambiguity detector for fast resolution of a 1/2cycle ambiguity in carrier phase during acquisition of an incoming GPS signal.
2. Description of the Prior Art
Global Positioning System (GPS) receivers are commonly used in applications where the GPS signal from one or more GPS satellites is temporarily blocked. For example, a mobile GPS receiver in an automobile may be driven through a location where the line-of-sight to the satellites is blocked by a building or thick foliage. When this happens the GPS receiver will lose lock on the GPS signal. Then, when the line-of-sight is restored, the GPS receiver must go through a set of pre-programmed routines to reacquire the GPS signal and continue with its application. It is desirable for most applications that the GPS receiver reacquire the GPS signal as quickly as possible so that the least operational time is lost.
Existing GPS receivers acquire or reacquire the GPS signal by synthesizing a carrier replica signal and a code replica signal based upon the phase and frequency of a local reference signal. The code replica signal has a one millisecond 1023 bit pseudo-random noise (prn) code matching the prn code of a desired GPS satellite. Carrier and code phase lock loops are used to adjust the phase and frequency of the replica carrier and code signals to match the phase and frequency of the carrier and code of the GPS signal. The GPS receiver is said to have acquired the GPS signal when the correlation between the replica signals and the GPS signal exceeds a predetermined correlation threshold. A pseudorange to the GPS satellite is determined from the phase of code replica signal that causes correlation. An apparent Doppler frequency and a GPS carrier phase observable, called "observable" because it is relative to the phase of the local reference signal, are determined from the adjustment that causes the correlation. The carrier phase observable has a 1/2cycle ambiguity because either a zero degree or a one-hundred eighty degree phase shift will provide the correlation. The GPS receiver computes its location, velocity, and time from the GPS message data, either received and stored previously or collected in the current GPS signal; and the pseudoranges, apparent Doppler frequencies, and carrier phase observables for typically at least four GPS satellites.
Existing GPS receivers resolve the 1/2cycle ambiguity by monitoring GPS data until a next preamble is recognized and verified and then detecting whether the preamble is non-inverted or inverted. The 1/2cycle ambiguity is resolved as zero degrees when data matching the preamble and the correct parity is detected and resolved as one-hundred eighty degrees when data matching an inverted preamble and an inverted parity is detected. Although this approach works, the preamble is only transmitted at about six second intervals so a time delay of about six seconds may be incurred from the time that the GPS signal is acquired or reacquired until the time that the next preamble is received and the 1/2cycle ambiguity in the GPS carrier phase observable is resolved. In applications such as real time kinematic (RTK) surveying or navigating a vehicle in an urban canyon, such delay in resolving the 1/2cycle ambiguity is inconvenient. There is a need for the GPS receiver to resolve the 1/2cycle ambiguity as quickly as possible.